For environmental protection, automobile engines are recently designed to have improved fuel efficiency, low emission and high power, so that engines tend to have high compression ratios and high load. However, higher compression ratios generally elevate combustion chamber temperatures, so that knocking easily occurs. A usual measure against knocking is the delaying of ignition timing (spark advance), but it makes it impossible to keep high thermal efficiency. Accordingly, investigation is conducted to lower the temperature of combustion chamber walls. To lower the temperature of combustion chamber walls, it is effective to lower the temperature of piston crown surfaces. To this end, it is most effective to dissipate the heat of pistons to cooled cylinder walls via compression rings. Thus, among the three basic functions of piston rings, a gas-sealing function, a thermal conduction function and an oil control function, the thermal conduction function is utilized. Because the thermal conduction function is affected by the thermal conductivity of ring substrates and surface treatment layers, ring shapes, etc., they should be optimized. Also, what should be taken into consideration in the selection of materials are, in addition to thermal conductivity, thermal sag resistance and fatigue resistance, which make it possible to keep ring characteristics even in a thermal environment at about 300° C.
In addition, when pistons are made of an aluminum alloy (simply called “aluminum” below), aluminum softens as the combustion chamber temperature is elevated, so that fatigue failure occurs in ring grooves of pistons due to the high-temperature impingement and sliding of compression rings, making likely the wearing of ring grooves and microwelding to compression rings. In view of this, the temperature of ring grooves should be lowered by using high-thermal-conductivity compression rings.
To meet the above demand, for example, JP 2009-235561 A proposes a composition comprising C, Si, Mn and Cr in proper ranges, for piston rings having excellent thermal conductivity and thermal sag resistance, which are usable as compression rings. However, for example, such targets as thermal conductivity of 35 W/m·K or more and a thermal sag ratio (loss of tangential force) of 4% or less would be difficult to achieve.
Further, not only excellent characteristics but also price competitiveness are required on automobile parts such as piston rings. Thus, cost reduction is an important object.